Magnetic Properties Of Non-magnetic Doping 8-hydroxyquinoline Small Molecules By First Principles

In the past few decades, spintronics is a hot topic in the research fields of electronics and condensed-matter physics because it uses both of the degree of freedoms, the electron charge and the electron spin, to design devices. Fundamental studies of spintronics include investigations of the spin polarization, spin injection, the spin relaxations and manipulation. Semiconductor spintronics is a promising candidate with many advantages including the nonvolatile data storage, the high speed of data processing, the high storage density, and the low energy consumption.Traditionally, the research of spintronics has been focused on the inorganic areas, organic spintronics is a new and promising research field, it is a cross subject of organic electronics and spin electronics. Organic semiconductors are, on the one hand, low in weight, cheap, mechanically flexible, and chemically interactive; On the other hand, organic materials have weak spin-orbit coupling and hyperfine interaction, extremely long spin relaxation time, high thermal stability and so on. Organic spintronics has been extensively studied experimentally and theoretically due to its great potential for a wide range of applications, such as, the organic light-emitting device (OLED), organic spin valves, display and the other organic spintronic devices.In 2002, Dediu et al. reported for the first time the spin injection and transport in sexithenyl(T6). In 2004, Xiong’s group reported a GMR response of 40% in a Co/AlQ3/La1-xSrxMnO3 spin valve. In the following researches, devices based on other organic small molecules and polymers have shown noticeable GMR. In 2011, Li Bin and Yoo et al. reported the V(TCNE)x/Rubrene/V(TCNE)x full organic spin valve using the organic polymer magnet V(TCNE)X as the ferromagnetic electrodes. Although the magneto-resistance values remain small, it indicates the potential applications of organic magnetic semiconductors in all-organic spintronic devices.Small organic molecules have caught a lot of research interestes in view of their unprecedented ability of self-assembly of ordered nanostructures. In particular, Metal 8-hydroxyquinoline complexes (TMQx, x= 2 or 3, with two or three 8-hydroxyquinoline ligands and a transition metal in the center) are widely used in organic light-emitting diodes (OLEDs) and organic solar cells in the light of their excellent optoelectronic properties. Among them, tris(8-hydroxyquinoline)aluminum (AlQ3) is a typical example. It has been used for spin polarized injection and transport as well as magnetoresistance effects. In 2008, Baik et al. found clear ferromagnetic behavior with a magnetic moment of ～0.33μB/Co in Co doped AIQ3 synthesized by thermal coevaporation of pure Co metal and AIQ3 powders. This spontaneous spin polarization in AlQ3-based materials provides a new route for spintronics devices and thus an exciting potential to combine magnetic ordering with semiconducting functionality and optoelectronic properties. Though our previous first-principles density functional theory (DFT) calculations show that an intrinsic ferromagnetism is likely to arise in Co-doped AIQ3, its dependence on a particular configuration and the complexity of magnetism in organic materials require a further check of this problem. Using nonmagnetic elements is an efficient way because secondary magnetic phase can be essentially avoided. We propose that this is not only an efficient way to study the intrinsic property of organic magnetism, but itself is useful for spintronics applications if this phenomenon is confirmed further by experiments. Besides, transition metal (TM) 8-hydroxyquinoline small molecules TMQx (TM= Cr, Mn, Fe, Co, Ni, Cu and Zn) may have intrinsic magnetic properties due to the partially occupied 3d states in TM atoms. 8-hydroxyquinoline-based complexes have not been broadly explored. Properties of some these molecules remain unclear with only very few pioneer research works showing that these molecules are wide bandgap semiconductors with paramagnetic or diamagnetic behaviors.In this dissertation, we studied the structure, electronic structure, and the magnetic properties of nonmagnetic doped 8-hydroxyquinoline complexes. The detailed contents and main results are given below:1. Structural, electronic and magnetic properties of 8-hydroxyquinoline-based small moleculesThe structural, electronic and magnetic properties of 8-hydroxyquinoline-based small molecules TMQx (TM= Cr, Mn, Fe, Co, Ni, Cu and Zn, and x= 2 or 3) are systemically studied by first principles calculations. For TMQ3 molecules, the TM-0 and TM-N bond lengths in different ligands are slightly different; TMQ2 molecules have highly symmetrical structures. These small molecules are spin-polarized semiconductors except for CoQ3, NiQ2, and ZnQ2. For TMQ3, the magnetic moments can be expressed as (6-n)μB (n= 3,4,5,6 for Cr, Mn, Fe, and Co, respectively). For TMQ2, the magnetic moments can be expressed as (8-n) μB (n is the number of 3d electrons of TM ions. For Mn, Fe, Co and Ni, n= 5,6,7 and 8 respectively) and (10-n) μB(n= 9 and 10 for Cu and Zn, respectively). The magnetism can be explained by the spin-polarized filling of splitting d orbitals on the basis of the crystal field theory. This provides theoretical insight for better understanding of 8-hydorquinoline-based complexes (TMQx).2. Spin polarization in diamagnetic tris(8-hydroxyquinoline)cobalt induced by nonmagnetic metalsThe magnetic and electronic properties of tris(8-hydroxyquinoline)cobalt (CoQ3) doped with Al or Cu have been studied by first-principles calculations based on spin density-functional theory (DFT). The results show that pure isolated CoQ3 molecule is nonmagnetic. Doping Al or Cu induces an exchange splitting between the majority and minority spins. The total magnetic moment is 1μB for these two systems. For Al doping, the electron transfer from Al to CoQ3 molecule, which is responsible for the magnetism. For Cu doping, the magnetic moment mainly originates from Co atom duing to the charge transfer from Cu atom to Co atom. The Cu atom also carries magnetic moments mainly associate with the O (p)-Cu (d) exchange interaction. Besides, the charge transfer from Al or Cu to CoQ3 also reduces the band gap of CoQ3. The finding of a spin polarization induced by the nonmagnetic element Al or Cu suggests that the doped CoQ3 is a promising magnetic organic semiconductor free of magnetic precipitates and may find applications in the field of spintronics.3. Spin polarization in diamagnetic tris(8-hydroxyquinoline)aluminum induced by Cu、 Nb and EuFirst-principles calculations based on spin density-functional theory (DFT) have been performed to study the magnetic properties of Cu-, Nb-and Eu-doped tris(8-hydroxyquinoline)aluminum (AIQ3). The results indicate that the entanglement between an electron transfer from Cu, Nb and Eu to AIQ3 and a structural distortion of the AIQ3 molecule gives rise to a strong spin polarization in this originally nonmagnetic molecule AIQ3. The calculated total magnetic moment is 1 μB,3 μB and 7 μB for Cu-, Nb-and Eu-doped AIQ3 molecule, respectively. For Cu-doped,0.4 μB carried by Cu and 0.6 μB by the AIQ3 molecule. For Nb-doped, the magnetic moments mainly originate from the 4d states of Nb atom. For Eu-doped, the magnetic moments mainly originate from the 4f states of Eu atom. After doping, AIQ3 molecule shows half-metallic properties for Nb-doped and ferrimagnetism for Eu-doped. This gives guidance for future material design for potentially organic spintronics applications.4. Charge-induced spin polarization in organic molecule bis(8-hydroxyquinoline)zincCharge injection in organic semiconductors is a key prerequisite for the realization of organic spintronics. The effect of electron injection ZnQ2 molecule has been simulated by ab initio density functional theory calculation. Our DFT calculations reveal that charge injection can induce a net magnetic moment in nonmagnetic organic molecule. The magnetic moment increases linearly with the injected charge. The spin density resides predominantly on the N atoms and C atoms in ZnQ2 molecule. The two Zn-N bond lengths are equal. Two N atoms have the same magnetic moments in ZnQ2 molecule. A symmetry of the Zn-N bond lengths leads to a symmetric contribution of injected charge over the molecule. Bader charge analysis indicates the transfer charges are localized uniformly on the two ligands, which lead to an symmetric distribution of spin density.